I have made Castile as HP and as CP. I've made it with the default settings in lye/
soap calculators (aka 'full water' to some) and with reduced water (aka 'water discount' or higher lye concentration). I've made it with regular OO and with pomace OO. EVOO is popular with some for Castile but I don't use it due to the high cost of the oil, but for some folks, EVOO is less expensive. I don't think the grade of OO has any real effect however (at least not on the cure time anyway.) I've made single lye Castile with sodium hydroxide (NaOH) and dual lye soap with 95% NaOH and 5% KOH (potassium hydroxide). I believe I've even made it with vinegar added (I'd have to double check to confirm, but I do recall at least one batch). I've made it plain without added color or fragrance and sort of fancy with colors and swirls and fragrances. I've made it in a traditional way with only hand stirring and I've made it using a SB (stick blender). I've made it with added salt (but not Zany's faux sea salt method.)
In my experience (and realize I've only been making soap just under 5 years) the longer cure time improves all of them. Any Castile I have has always improved with age; every last one of them. So when you read about a 15 month cure (or whatever long cure you read, as it can vary with source) I would support that.
A higher lye to water ratio, although it makes the soap harder faster, doesn't reduce the time required to improve the soap, or 'cure time' as it is often called. Like
cmzaha, I like the [40% lye] as it sets up faster for molding and the bars don't warp at all while they cure. The default water setting in lye/
soap calculators (or 'full water' as some call it) has so much water in the bars that I found that if I cut bars with the common rectangular shape, the bars warp during cure, but that can happen with other recipes as well, so it's not just with Castile. That's one reason I started cutting into cubes are other larger chuncky sized shapes because I don't like warped looking soap. But with a higher lye to water ratio, I can cut into any shape and not worry about my soap warping during cure.